1. Technical Field
The present invention relates to a light detector.
2. Background Art
In order to reduce traffic accidents or the like, vehicles equipped with a collision prevention system or the like are being developed. In such a system, sensors such as cameras, a millimeter wave radar, or the like for monitoring the external road environment are used.
A stereo camera has a relatively wide field of view and high spatial resolution, but the distance precision is significantly reduced at a far distance. On the other hand, the millimeter wave radar can detect a target which is about 200 m away, but the millimeter wave radar has a narrow field of view and low angular resolution.
In contrast, an optical rangefinder based on a time of flight (TOF) method has high spatial resolution (angular resolution) and can measure a distance with a wide field of view and a long distance. Because of this, such a rangefinder can improve precision and enhance robustness to detect a driving path and obstacles, and thus, functions of the safety system can be expanded. For example, if an obstacle at a far distance can be detected with higher position precision, a warning can be activated earlier. In addition, if a shape of a parked vehicle or the like can be detected with high precision, it can be reliably judged if a collision will occur.
In such an optical rangefinder based on TOF, an avalanche photodiode (APD) or a PIN photodiode is typically used as a light detector. When a photon falls on the APD, an electron-hole pair is generated, the electrons and the holes are accelerated with a high electric field, causing impact ionization, and new electron-hole pairs cause impact ionization one after another in a manner similar to an avalanche. Because the sensitivity is improved by this internal amplification, the APD is often used, especially in a case where a long-distance detection is desired. Operation modes of the APD include a linear mode in which the APD is operated with a reverse bias voltage at a voltage slightly less than a breakdown voltage, and a Geiger mode in which the APD is operated with a reverse bias voltage at a voltage of greater than the breakdown voltage. In the linear mode, a ratio of the electron-hole pairs which disappear (which exit from the high-electric field region) is higher than a ratio of the electron-hole pairs which are generated, and thus, the avalanche stops naturally. An output current is approximately proportional to the amount of incident light, and the device is used for measurement of the amount of incident light. In the Geiger mode, because the avalanche phenomenon can be caused with incidence of only a single photon, the device is also referred to as a single photon avalanche diode (SPAD).
An optical rangefinder based on TOF can also output brightness information in addition to the distance information. The detected light includes light which is illuminated by the rangefinder and reflected on an object, ambient light such as sunlight which is reflected on an object, and light which is radiated from an object. In the case of a light detector that outputs a value which is approximately proportional to the amount of incident light, the peak of the light signal can be assumed to be the reflection, and the reflection can be identified by extracting the peak. The peak value of the light signal can be corrected with a measured distance, to determine reflectivity of the target. By contrast, the light other than the illuminated light; that is, the disturbance light, can be measured from the output of the light detector during a laser stopping period (JP 2011-247872 A).
On the other hand, in the case of the light detector of a photon-counting type, the brightness information can be measured from the TOF histogram. A total of the histogram values is a full amount of detected light, and the peak value is the reflected light of the illuminated light. In addition, information of the disturbance light can be obtained from the output of the light detector during the laser stopping period (JP 2010-91378 A).
When the light detector is used outdoors, a wide dynamic range is required. A brightness varies a lot in the outdoor environment, because the illuminance exceeds 100,000 lux in fine weather during daytime and the illuminance is about a few tens of lux under a street lamp during the nighttime. Since variation of the target reflectivity should be also considered, a dynamic range of about 6 orders of magnitude is required. When the amount of light is detected with the photon count, the number of counts is approximately proportional to the amount of light when the amount of light is low. When the amount of light is increased, the photons incident interval gets shorter than an output voltage pulse width. Then, a plurality of voltage pulses may be merged and thus, the number of counts could be reduced. Therefore, as shown in FIG. 8, the number of counts cannot monotonically increase in accordance with the amount of light, and the amount of light cannot be accurately measured.
Thus, the light detector is repeatedly reset by a resetting mean, it is detected if there is one or more incident photons between the reset pulses, and the detected binary result is accumulated for a predetermined period. With this process, because the number of necessary bits for the counting is reduced, the dynamic range can be enlarged (JP 7-067043 A). There is also another disclosed method in which an analog detection signal of a photon counter is converted to a digital signal. When the digital signal is greater than or equal to a threshold, the signal is simply forwarded to a later stage of a counting circuit. When the digital signal is less than or equal to the threshold, a predetermined value is sent to the later stage. In the counting circuit, an amount of light is calculated based on an integration of the waveform of the obtained detection signal until the amount-of-light measurement is completed (JP 2012-37267 A).
In the related art (JP 7-067043 A), when the light detector is reset, the time required for the reset is not considered, but actually, the reset requires a certain amount of time, which is the time required for recharging a parasitic capacitance of the photodiode with charges and biasing the photodiode to a predetermined voltage. The reset time becomes a dead-time in which photons cannot be detected, and, as shown in FIG. 9, when the sampling interval is shortened, a ratio of active time during which the photons can be detected; that is, an efficiency of detection time, is reduced. On the other hand, if the sampling interval is elongated, a ratio of the active time during which the photons can be detected can be increased, but the detection result is easily saturated, and, consequently, the dynamic range is reduced.
In addition, in the related art (JP 2012-037267 A), because an output waveform integration is also varied when the detection sensitivity of the light detector varies with temperature, the amount of light cannot be accurately detected. In addition, the noise level also varies with temperature, and, thus, the threshold cannot be suitably defined.